Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which transmit an ultrasound beam and then receive the beam reflected from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g. , by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate, respective time delay (and/or phase shift) and gain to the signal from each receiving transducer element.
Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
The response of an ultrasound system focused in a given direction is peaked around that direction. The response is not zero, however, for other directions. In some ultrasound images, the reflected signal coming from a given direction is contaminated or obscured by spurious reflections from a bright object, e.g., bones, in a nearby direction. Therefore it would be advantageous to adapt the response of the ultrasound imaging system to reduce artifacts caused by such interfering reflectors.